A band pass filter (BPF) is an electronic component that is used for filtering out unwanted frequencies for a connected device. That is, a band-pass filter allows frequencies within a certain range and rejects, or attenuates, frequencies outside that range.
For example, FIG. 1 shows a graph of frequencies passing through an ideal BPF. The bandwidth that is permitted to pass though, i.e., the passband as indicated on the graph, is the range of frequencies that extends from a minimum frequency of f1 to a maximum frequency of f2. The amplitude of the passband, as shown on the vertical axis of the graph, is measured in decibels and indicates the amount of insertion loss, or loss of signal, of the transmitted frequency. An ideal BPF contains zero insertion loss.
The graph indicates a perfectly vertical rejection curve, which allows all of the desired frequencies, or the passbands, to successfully pass through the filter while preventing all of the unwanted frequencies, or the stopbands, from passing through. A perfectly ideal vertical curve creates a distinct cut off point between the stopbands and the passband.
The requirements of a desirable BPF include both a low insertion loss and a steep rejection curve. However, unlike an ideal BPF, an actual BPF is unable to create a perfectly vertical rejection curve or zero insertion loss. FIG. 2 shows a graph of frequencies of an actual, non-ideal, BPF. The rejection curve is shallow, both between the minimum frequency and the passband and the maximum frequency and the passband. Additionally, there is insertion loss of allowed frequencies within the passband, e.g., 3 decibels.
Many currently available BPFs employ one or more resonators having resonance of certain frequencies. Signals with frequencies close to the resonant frequencies pass through the filter, while signals farther away are blocked. In the related art, three main designs of current resonators include: (a) resonators based on capacitors and inductors; (b) resonators based on surface and bulk acoustic wave filters (known as SAW and BAW filters); and (c) resonators based on a cavity in a dielectric material.
FIG. 3A shows a diagram of an LC resonator-based BPF using several connected inductors (L1, L2, L3 and L8) and capacitors (C1, C2, C3, C8, and C9). Connected components C1 and L1 and other similar capacitors and inductors are the basic components of LC-resonators. These BPFs often employ multiple LC-resonators that are coupled together, as one resonator is usually insufficient to provide the required or desired rejection curve steepness. Thus, several consecutive and connected resonators are often used together. However, as illustrated in the graph shown in FIG. 3B, the rejection curve produced by such resonators is shallow, resulting in a gradual transition between the stopband and the passband, permitting some unwanted frequencies to pass through while also blocking some desired frequencies, resulting in an inaccurate BPF.
FIGS. 4A and 4B show a SAW and a BAW type of BPF, respectively. These filters are popular designs used in modern wireless communication devices due to their high rejection rate of unwanted frequencies (˜30-50 dB in proximity of ˜50 MHz to the passband). Namely, they produce, as shown in FIG. 4C, a steep rejection curve. However, the insertion loss created by SAW and BAW filters is often undesirably high. Further, additional limitation exists. Specifically, SAW type filters are only effective for frequencies of up to ˜3.5 GHz, but are not available for higher frequency bands that are currently in development for many mobile devices (e.g., 6 GHz, 28 GHz, etc.). Further, BAW type filters can be very expensive to produce and are likewise not effective for the 28 GHz band. Additionally, the passband created by SAW and BAW types of BPF is relatively narrow (between 70 MHz-100 MHz or similar) and cannot be adjusted.
The third type of BPF is based on a cavity in a dielectric material, as shown in FIG. 5A. These filters allow for both a steep rejection curve and low insertion losses. However, due to the physical structure required for optimal performance, such filters are large in size. Evidently, as demonstrated in FIG. 5B, the third type of BPF are often too big for use in many applications, such as smartphones, tablets, wearables, and the like.
In addition to the physical size, the narrow bandwidth is a limiting factor for using a conventional BPF in mobile telephones or smartphones. A mobile telephone should operate in the entire frequency band of radio frequency (RF) signals designed to be transmitted and received by the telephone. In modern communication standards, the frequency band (bandwidth) of mobile (cellular) telephones may include a wide range of frequency. Typically, such a frequency range is between 1 GHz and 7 GHz while operating at a multi-band frequency. As discussed above, conventional BPFs cannot meet this demand. In addition, the insertion loss causes poor transmission and reception of RF signals. Thus, a smaller version of a BPF with low insertion loss, wide passbands, and steep rejections curves is desirable.
It would therefore be advantageous to provide a BPF that would overcome the limitations noted above.